The River Tevvyn was a river in the west of Prydain, one of the waterways that flowed into the Great Avren. Its source seems to have been in the Hills of Bran-Galedd in the northwest; it then ran southward through the Forest of Idris to empty into the wide, bay-like mouth of the Great Avren. The Tevyn's waters were brown and turbulent.
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The Tevvyn was the river which Taran and the Companions had to cross while bearing the burden of the Black Crochan. It was here, too, that Ellidyr tried to drown Taran and kill the other companions -- and almost succeeded.

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